Predation pressure on calanoid copepods varies among the diverse habitats they
occupy, selecting for copepods with different anatomical and behavioral traits. Calanoid
copepods from seven families in three superfamilies from estuarine, neritic and oceanic
habitats were exposed to a controlled near-field hydrodynamic stimulus and their escape
reactions were recorded using high-speed videographic techniques. Myelinated axons
have been found in certain species of the more recently-evolved calanoid superfamilies.
Copepod representatives from these superfamilies were expected to have shorter response
latencies than species from more ancestral superfamilies due to the increased conduction
speed of nerve impulses in myelinated neurons. Using frame-by-frame playback and
computerized motion analysis techniques, response latency, jump speed, and acceleration
were measured. Kinetic performance of copepods was highly variable, with mean escape
speeds ranging between 100 – 250 mm s-1 and accelerations of 49 – 230 m s-2. Minimum
behavioral response latencies of 2 ms were recorded for both myelinated and nonmyelinated
calanoids. The shortest mean response latency of 3.20 ± 0.20 ms was
recorded for the myelinated Mesocalanus tenuicornis. There was no significant
difference between the response latencies of copepods from the myelinated and nonmyelinated
superfamilies. Furthermore, no relationships were found between copepod
latency and size for either myelinated or non-myelinated species. Although previous
research may suggest that myelin may shorten the response latencies of certain calanoid
species, non-myelinated copepods are also capable of responding rapidly, within as few
as 2 ms, to hydrodynamic stimuli and produce similar kinetic performance to myelinated
species.
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